1. Field of the Invention
The present invention relates in general to automatic stenciling apparatus and more specifically to such apparatus for stenciling multi-color designs on cylindrical objects having a registration apparatus adapted for use with a laterally moving silk screen.
2. Description of the Prior Art
Automatic stenciling apparatus are known in the art for rotatably supporting a bottle, mug or other cylindrical object against the surface of a silk screen having a prepared design to cause the bottle to rotate in registration with the design. In the process of stenciling cylindrical or nearly cylindrical objects, the frictional driving engagement between the object and the silk screen causes rotation of the object and produces a satisfactory image. However, such means of maintaining registration between the object and the screen does not prove satisfactory for multi-colored stencils for which a precise registration between the screen and the object is required.
To provide more precise registration between the object and the screen, it is known in the art to provide a positive driving connection between the screen frame and the mount supporting the object such that the translation of the screen rotates the object in synchronism, thereby preventing relative slippage between the surface being stenciled and the screen. Examples of prior art mechanisms having such a positive driving connection are shown in U.S. Pat. Nos. 3,090,300; 3,260,194 and 4,111,118. One of the more currently used type of systems is detailed in U.S. Pat. No. 3,260,194 and includes a pinion gear attached to one end of a rotatable shaft and a device for holding the object to be printed attached to the other end. Meshed with the pinion gear is a rack, mounted horizontally with a linear bearing at each end. Vertical rods attached to the back of a screen assembly engage these linear bearings allowing the rack to move laterally in synchronization with the screen, while remaining meshed with the pinion when the item holder is lowered from the screen for loading and unloading. As the screen and rack move laterally, the rack transfers the horizontal linear motion of the screen to rotational motion of the object being printed, allowing the image in the flat horizontally mounted screen to be transferred to the curved surface of the item.
Critical factors involved in registering multi-color printing include closely matching the rotation rate of the item with the linear rate of the screen, a 1:1 ratio between their surfaces being ideal and critical, and a smooth, accurate rotational motion with no slop or back-lash to introduce left and right variation into the relative location of the imprint colors.
The current rack and pinion systems require an extensive inventory of expensive and in many cases custom made pinion gears to accommodate items of differing diameters. The minimum increment of adjustment is one gear tooth, typically 1/10th of an inch. Multicolor printing requires registration tolerances as low as 1/100th of an inch, so the rack and pinion gear system is generally not accurate enough to product high-quality multi-color imprints, especially on coffee mugs and plastic bottles.
In new rack-and-pinion gear systems, a certain amount of clearance or space must be provided between the mating gear teeth to facilitate smooth operation, or binding and rumble or rough operation will result. As rack and pinion gears wear, the clearance or back-lash increases, and there is no method of adjusting or compensating for this wear (other than to replace both the pinion and rack gears), so these systems tend to develop inaccuracies as they are used. These factors along with the high cost of maintaining an inventory of many different sized gears, are addressed by the present invention.
Although the invention disclosed in U.S. Pat. No. 3,260,194 provides an assembly that improves the registration between the movement of the screen and the article, it is not adapted to provide the precise registration necessary for the stenciling of multi-color designs on articles or to avoid the back-lash that is inherent in a rack and pinion system.